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Short Term Stability
If aging is the gradual change in frequency over the period of days, months and years, short term stability is the change of frequency typically exhibited per day. All of the variable mechanisms that contribute to longer-term aging contribute to variations in short term stability. Changes in short term stability are typically in parts per billion, 10-9 per day.

Large changes in short term stability are called "pops" and "jumps" . These changes in short term stability also contribute to "retrace" or hysteresis of the resonator frequency over temperature.

Manufacturing processes are developed and selected to minimize short-term changes in frequency.

Causes of short term instabilities are:
  • Temperature fluctuations
  • Thermal transients
  • Activity dips at oven set-point
  • Johnson noise - thermally induced EMF in a resistive element
  • Vibration
  • Acoustic losses - Q
  • Fluctuations in the number of adsorbed molecules
  • Changes in interfaces between quartz, electrodes, mount & bond

Short-term stability measurements can be derived by each of the following methods:

  • Two sample (Allan) variance
  • Spectral density of fractional frequency fluctuation
  • Spectral density of phase fluctuation
  • SSB phase noise to carrier ratio

Aging and Short Term Stability

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